Scale whole object in normal orientation
In Edit Mode, create  a Custom Orientation from the face. (That's the '+' in the orientation dropdown at the top of the view, in 2.8 .) Check 'Use After Creation'..Then back in Object Mode, use it, with SZZ

However, it's probably better to A select the whole mesh in Edit mode, and scale that, because non-uniform scaling of the object (giving the object a transform matrix including a non-uniform scale) often interacts in undesirable ways with modifiers and constraints. Either that, or when you're done scaling in Object mode, CtrlA, bake the scale into the mesh by applying it.